[time-nuts] GPS/UTC time

[Bob Camp]

Hi
> On Jul 4, 2015, at 3:18 PM, Brian Inglis <Brian.Inglis at SystematicSw.ab.ca> wrote:
> 
> On 2015-07-04 07:36, Jim Lux wrote:
>> On 7/3/15 9:45 PM, Brek Martin wrote:
>>> I feel like I missed “The Big Thing” in time keeping land. I should have watched my Ublox LEA-5T.
>>> What is the difference if it is in the reporting mode for GPS or UTC time?
>>> If they skip a second UTC, surely the GPS time isn’t run incorrectly forever.
> 
>> Who's to say which is "correct", UTC or GPS?
>> GPS time is derived from TAI; both are monotonically increasing, continuous, and constant rate.  These are nice attributes for something you're going to use for time stamping, or controlling.  No gaps, no jumps, etc.
>> UTC (and local civil time, and GMT, etc.) have leap seconds, to adjust the time scale to the motion of the Earth;  so that the sun is highest at noon (after accounting for the equation of time).
>> While that's somewhat convenient, I doubt anyone would really object to noon being a few tens of seconds away from the zenith crossing when standing on the line. TAI is ahead of UTC by 36 seconds.  They add a leap second every year and a half, so I guess in 100 years, we'll have drifted some minute or so away.  (the earth has slowed down in the last 200 years.. a day is now 86400.0015 seconds long, although it's faster now than it was in the 70s, when it was 86400.003 seconds)
>> http://tycho.usno.navy.mil/leapsec.html
>> (the Japan earthquake in 2011 sped the earth up by 1.8 microseconds/day.  The Sumatra quake on 26 Dec 2004 had a bigger effect: 6.8 microseconds)
> 
> By my calculations, that should mean the earth rotated 3mm farther/day at the equator after Sumatra.
> Anyone know if, or how much, these perturbations affect the orbit of the earth or other planets, or where to find out?

The simple answer is “not at all”. The mass of the earth did not change so the “gravity at a distance” did not change either. The more complex
answer is that of course it changed things. Since we can’t even detect the change on earth, the change elsewhere is even more undetectable. 
Given that the whole solar gravity field is determined by a *lot* of things and that we do not have a full data set on all of those things, you get down
into the noise on the solutions well before things like this are an issue. 

Bob

> 
> -- 
> Take care. Thanks, Brian Inglis
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